Salt uptake and evapotranspiration under arid conditions in horizontal subsurface flow constructed wetland planted with halophytes

The use of halophytes in constructed wetlands (CWs) for phytodesalination has been recently suggested as a strategy for dealing with excess salt, which causes land degradation and has detrimental effects on agricultural productivity. Salinity in drylands is enhanced by strong solar radiation and increased evapotranspiration, and poses additional challenges for decentralized water-treatment systems such as CWs. We tested the potential use of a horizontal subsurface flow CW (HSSF CW) planted with the halophyte Bassia indica to polish treated municipal wastewater quality and reduce salinity, using a pilot system containing eight flow cells. The system was operated under continuous flow of synthetic, secondary level-treated wastewater at a rate of 480-660 ml/h, which resulted in a retention time of approximately 2 days. Water fluxes, and nutrient and salt concentrations in the water were measured and compared to those in flow cells without plants. Plant height was measured periodically and biomass and salt concentrations were recorded at harvest. Removal of biological oxygen demand, and reduction in fecal coliforms and turbidity were evident and similar in planted and non-planted cells. Significant salt uptake was measured in the planted cells, with the amount of Na+ uptake directly related to plant biomass; however, the overall salinity of the treated wastewater increased despite salt uptake by B. indica due to enhanced evapotranspiration. This study reaffirms the notion that halophytes in CWs have the potential capacity for direct uptake of salts. Nevertheless, efficient salt phytoremediation in arid and semi-arid climates will require specific application of plants and relatively short hydraulic residence time to minimize evapotranspiration.